1. Field of the Invention
This invention relates, in general, to means and methods for improved bonding of wires to substrates, more particularly to the design and use of an improved bonding tool and bonding capillary to produce improved wire bond interconnections in electrical devices, especially semiconductor devices, and to electrical devices having improved bonds thereon.
2. Description of the Prior Art
Wire bonding is much used to provide lead wires and/or interconnect wires for electrical devices, especially semiconductor devices. The wires may be bonded to contact pads by use of pressure, heat, vibrational energy, or a combination thereof. Ordinarily, the wire welds to the contact pad without the use of solders or fluxes by the intimate mixing of the pad material and the wire material at the interface.
"Thermal compression" refers to bonding by means of the application of heat and pressure. "Ultrasonic" refers to bonding achieved as a result of the scrubbing action made possible through the application of ultrasonic vibrational energy to the bond interface. "Thermosonic" refers to bonding achieved by the use of a combination of heat, pressure, and ultrasonic energy.
Gold, aluminum, silver, and copper in pure or alloy forms, are typical bonding wire materials. For semiconductor device applications, wire dimensions are usually 0.5 to 25 mils (0.013-0.64 mm), 0.7 to 2 mils (0.018-0.051 mm) being most common. Contact pads on semiconductor devices are frequently formed from aluminum, gold, copper, silver, or other metals in pure or alloyed form. Contact pads on device mounting frames or package leads are frequently made from gold, aluminum, copper, silver, nickel, iron, and/or common alloys such as, for example, Kovar (Kovar is a registered trademark). The various contact pad materials may be created by evaporation, plating, sputtering, screen printing, or formed from bulk material, each of which produces different surface conditions and bonding properties.
As used herein, "substrate" refers to a contact pad or other region to which a wire is or is to be bonded.
Bond strength refers to the force required to destroy the bond by breaking the bonding wire or separating it from the substrate. Bond strength depends on the wire size, wire material, contact material, bond area, bond shape, and bonding method. It is generally desired that the bond strength be greater than the tensile strength of the wire so that failure occurs as a result of wire breakage rather than bond separation, since this provides the maximum bond strength for a given wire size and material. Achievement of this condition, however, has not always been possible in the prior art, particularly with materials such as copper to which it has previously been difficult to make a proper bond.
Thus, a need continues to exist for a means and method for achieving wire bonds having improved pull strengths, particularly on copper or copper alloys so as to avoid the use of precious metals on bonding surfaces, and for electrical devices having wire bond connections whose strength is determined by the tensile strength limit of the wire rather than the bond interface.
Accordingly, it is an object of the present invention to provide an improved bonding means giving improved bond pull strengths.
It is an additional object of this invention to provide an improved bonding means giving a bond region of substantially uniform thickness.
It is a further object of this invention to provide an improved bonding means having two regions on the bonding surface, one for providing the bonding action, and a second non-planar with the first for providing for cut-off of the bond wire.
It is an additional object of this invention to provide an improved process for producing single or multiple wire bonds to contact regions of substrates.
It is a still further object of this invention to provide electrical devices having improved wire bonds.